Relatively pure metal oxides are produced by thermal decomposition of precursors and deposition of the resulting oxides. The precursor may take the form of a vapor, or may be carried by a vapor. It may be decomposed by either flame hydrolysis or pyrolysis.
One such process is production of fused silica by hydrolysis or pyrolysis of a silica precursor. Commercially, this is an application of flame hydrolysis involving forming and depositing particles of fused silica which melt to form large bodies (boules). Such boules may be used individually, may be finished and integrated together into large optical bodies, or may be cut into small pieces for finishing as lenses and the like. In this procedure, the precursor is hydrolyzed and the hydrolyzed vapor is passed into a flame to form particles of a fused silica. The particles are continuously deposited, for example, in the cup of a refractory furnace where they melt to form a solid boule.
Essentially pure fused silica finds many diverse applications. However, it does have a small positive coefficient of thermal expansion (CTE) that can make it undesirable in some instances. U.S. Pat. No. 2,326,059 (Nordberg) describes a fused silica doped with 5-11% by weight titania (TiO.sub.2). These TiO.sub.2 -doped glasses have CTEs lower than pure fused silica with the potential for a CTE that approximates 0.
Originally, chlorides of silicon and titanium were employed as precursors. Recently, primarily for environmental reasons, chloride-free precursors have been proposed. Specifically, a siloxane, octamethylcyclotetrasiloxane (OMCTS), and a titanium alkoxide, titanium isopropoxide, Ti(OPri).sub.4, are commercially employed.
The precursors are separately converted to vapor form and carried to mixing manifold by a carrier gas, such as nitrogen. The mixture passes, via fume lines, into a flame where the precursors are converted into SiO.sub.2 --TiO.sub.2 particles. These particles are collected in a refractory furnace where they melt to form a solid boule.
With the changeover to the new precursor materials, problems have been encountered. These problems are manifested largely by material build-ups in the vapor delivery system. The build-ups cause erratic operation, and consequent furnace upsets. Ultimately, they require shut down of the vapor delivery system for cleaning.
It is a basic purpose of this invention to provide an improved method of producing a TiO.sub.2 -doped fused silica.
Another purpose is to minimize material build-ups that occur in the vapor delivery system during operation.
A further purpose is to improve the quality of the TiO.sub.2 -doped fused silica produced.
A still further purpose is to lengthen the time of a production run before it is necessary to shut the operation down for cleaning purposes.